Stem cells (for example, hematopoietic stems cells, or “HSCs”) provide many potential therapeutic uses in vivo. Stem cells' ability to differentiate into a variety of mature cell types indicates that undifferentiated stem cells may be clinically useful, for example, in treating disease both malignant (e.g., chronic myelogenous leukemia, acute myelogenous leukemia) and non-malignant (e.g., severe aplastic anemia, inherited metabolic disorders). A problem in using human stem cells in vivo, however, is that while stem cells may differentiate into a variety of mature cell types, the lifespan of a specific human stern-cell cell culture is limited by the cell line's ability to “self-renew” or propagate new undifferentiated stem cells (called “self-renewal”). Thus, the art has sought a way to increase the lifespan of human stem cell cultures or cell lines, by increasing self-renewal.